Decking boards, siding and a number of other products are made by extruding a substrate and then applying a cap layer or layers on at least one surface, usually the top surface of the substrate. Optically clear polymers have been used for making a transparent cap layer. It is known that streaks can be created in the cap layer or cap layers by inserting color pellets into the cap layer material. During the extrusion process the color pellets melt creating streaks in the cap layer and providing a variegated appearance.
U.S. Pat. No. 4,048,101 to Nakamachi discloses the manufacture of opaque articles having a variegated pattern of streaks. The variegated pattern is produced by extruding a foamed polymeric matrix material together with polymeric accent materials having colorant additives, which form accent color streaks in the polymeric matrix material. The polymeric accent materials have a lower melt index and a larger particle size compared to the foamed matrix material, such that its melt flow rate is lower than that of the matrix material. The accent material and the matrix material are resinous compositions of the same chemical type, to assure bonding therebetween.
U.S. Pat. No. 5,387,381 to Saloom discloses a method for making an opaque cap layer having a simulated wood-grain appearance for coextrusion with a polyvinylchloride (PVC) substrate. The cap stock material combines a polymeric matrix with accent color pellets consisting essentially of acrylics or polycarbonates having a higher deflection temperature and a larger particle size compared to the polymeric matrix. During an extrusion cycle, the polymeric matrix undergoes melt flow, and the accent color pellets undergo melt flow later in the extrusion cycle, which produces accent color streaks in the extruded cap stock matrix.
U.S. Pat. No. 5,866,054 to Dorchester discloses an opaque cap stock manufactured with a PVC polymer matrix together with accent color pellets comprised of, acrylic resins blended with poly-.alpha.-methyl styrene (PMS) resin. The PMS resin in the pellets has a higher vicat softening point temperature and a higher melt flow index relative to the acrylic resins in the same pellets. Varying the proportion of the PMS to the acrylic resins in the pellets results in a corresponding, soft streaking (gradual color change) or hard streaking (abrupt color change) in the polymer matrix.
United States Published Patent Application No. 2005/0003221 A1, to Walrath, discloses an optically clear cap stock manufactured from a matrix resin of methacrylic acid polymer or other essentially clear, ultraviolet resistant, matrix materials, and streaker pellets of a polymeric material compatible with the methacrylic matrix resin but having a higher melting point. The matrix resin and streaker pellets are fed into an intake end of an extruder screw feed. The screw feed has four temperature stages, such that the transparent matrix material melts and flows before the streaker pellets become melted within the last temperature stage. Then, the melted materials are extruded to form a transparent cap stock having accent color streaks in a transparent matrix. Further, the cap stock is coextruded with an underlying polymeric substrate that is visible through the transparent cap stock. The cap stock has a heat deflection temperature and a glass transition temperature similar to that of the coextruded substrate.
U.S. Pat. No. 7,987,885 B2 to Zhou et al. discloses in FIG. 1 an extrusion line for making a reinforced structural plastic profile such as a decking board. This production system has filaments which are drawn into a die 18. A first extruder 24 injects a resin into the die to surround the filaments and form the substrate. A second extruder 35 supplies a second resin which forms the cap layer over the substrate. At column 4, line 64, through column 5, line 4, the patent teaches that a fourth resin could be applied in a subsequent coextrusion or extrusion coating step.
U.S. Pat. No. 8,955,281 B2 to Pietruczynik et al. discloses a building product that can be used for a decking board which has a hollow core over which a cap stock layer is applied. The patent teaches that additional layers can be coextruded to form a multilayer article. See column 12, lines 58-66. The patent also teaches that a variegated pattern may be created by using pellets of different viscosities.
It is quite common to emboss the surface of the decking board or other product containing the cap layer. To achieve a consistent embossing pattern on the entire embossed surface it is critical that the surface which is being embossed be smooth across the entire surface. Even small depressions or projections on the surface can change the appearance of the embossing pattern. Such depressions and projections can also cause uneven surface wear such that the appearance of the capped surface will change over time. For that reason the art has not applied streaks onto the top of the cap layer but always has created the streaks within the capping layer.
However, if streaks could be applied onto the cap layer without interfering with the embossing process or creating uneven wear in the finished product then one would have greater control of the width and locations of the streaks in the final product. This control would enable the manufacturer to make variegated products which all have the same pattern on the variegated surface.